1. Field of the Invention
The invention relates to a catalyst for the catalytic ammoxidation of alkanes, more specifically C3 to C5 paraffins, such as propane or isobutane, to the corresponding α,β-unsaturated mononitriles, e.g. acrylonitrile and methacrylonitrile. The disclosed catalyst can be utilized also for the ammoxidation of olefins, such as propylene and isobutylene, to the same nitriles. In addition, the catalyst may be used in the ammoxidation of xylenes and methylpyridines to the corresponding mono- and dinitriles.
The invention is directed also to making the catalyst by precipitating, drying and calcining the catalyst precursor to produce active phases with the right proportion. The invention is directed also to using the catalyst in a process for catalytic ammoxidation of alkanes.
2. Description of the Prior Art
Nitriles, such as acrylonitrile and methacrylonitrile, are useful as intermediates for the preparation of fibers, synthetic resins, synthetic rubbers, and the like. One method for producing such nitriles is a gas phase reaction at a high temperature of an olefin, such as propene or isobutene, with ammonia and oxygen in the presence of a catalyst. Known catalysts for conducting this reaction include Mo-based and Sb-based oxides. However, in view of the price difference between propane and propene or between isobutane and isobutene, there are advantages for development of a method for producing acrylonitrile or methacrylonitrile by an ammoxidation reaction with a lower alkane, such as propane or isobutane, as a starting material, which is catalytically reacted with ammonia and oxygen in a gaseous phase in the presence of a catalyst. The majority of effective catalysts reported for propane ammoxidation generally contain vanadium oxides in combination with oxides of antimony and/or molybdenum.
A number of patents assigned to the Standard Oil Company disclose V-Sb oxide catalysts promoted with different elements and various procedures for preparation of those. For instance, U.S. Pat. Nos. 4,746,641 and 4,797,381 disclose paraffin ammoxidation catalysts that contain tungsten in addition to vanadium and antimony. U.S. Pat. Nos. 4,871,706 and 4,873,215 disclose V-Sb oxide catalysts with tungsten and phosphorus. U.S. Pat. No. 5,079,207 discloses a catalyst with tellurium or bismuth in addition to vanadium-antimony. Bismuth is also optionally present in the vanadium-antimony based catalyst claimed in the U.S. Pat. No. 6,162,760. U.S. Pat. No. 5,336,804 discloses vanadium-antimony based catalysts in which bismuth is always present and iron, gallium, indium and mixtures thereof are optionally present.
A molybdenum-based catalyst for propane ammoxidation is the Mo—V—Nb—Te oxide system is disclosed in the U.S. Pat. No. 5,281,745. This catalyst may optionally contain tungsten, bismuth and other elements. U.S. Pat. No. 4,978,764 discloses Bi—Fe—Mo based oxide catalysts in which bismuth must be present and tungsten is optionally present. U.S. Pat. No. 5,470,815 describes In—Ga—Bi—Mo based oxide catalysts that contain always bismuth and optionally tungsten. U.S. Pat. No. 4,760,159 discloses a propane ammoxidation catalyst having the formula BiaVbLlMmTtOx, in which Bi, V, M (selected from among Mo, W, Cr, Ge and Sb) and oxygen are necessarily present. From a number of examples set forth in this patent, it comes that best selectivities to acrylonitrile are obtained on catalysts containing vanadium, bismuth and molybdenum and, optionally, another metal such as chromium, potassium, zinc, cesium or antimony. One example uses a catalyst based on vanadium, bismuth and tungsten of the formula Bi1V0.7W0.5Ox and deposited onto a silica/alumina mixture in an amount of 50% by weight. This catalyst, however, produces acrylonitrile with the selectivity considerably lower than that attained on catalysts containing molybdenum.
U.S. Pat. Nos. 4,111,983 and 4,289,654 claim an improved process for acrolein oxidation to acrylic acid using Mo—V based oxide catalysts, which necessary contain tungsten. Bismuth is not mentioned even to be an optional element. U.S. Pat. No. 6,693,059 describes a method for preparing catalyst for propane oxidation to acrylic acid having the formula WaVbXxYyOn, wherein X is at least one element selected from the group consisting of Te, Bi, Sb and Se. The examples listed in this patent use catalysts that contain tellurium along with tungsten and vanadium. None of them contains bismuth. Also, there is no mention of the usefulness of these catalysts for ammoxidation reactions.
U.S. Pat. No. 6,514,902 discloses a process for producing acrylonitrile or methacrylonitrile from propane or isobutane with a catalyst containing at least molybdenum, vanadium and antimony and optionally niobium, tungsten, chromium, titanium, tantalum, zirconium, hafnium, manganese, rhenium, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, copper, silver, zinc, boron, gallium, indium, germanium, tin, tellurium, phosphorus, lead, bismuth, rare earth elements and alkaline earth metals wherein the catalyst is made by subjecting a raw material mixtures solution or slurry to an oxidation treatment.
U.S. Pat. No. 4,883,895 discloses a process of catalytic ammoxidation of propane and isobutane to acrylonitrile and methacrylonitrile with a first catalyst composition containing vanadium, phosphorus, tungsten, one or more of iron, cobalt, nickel, chromium, lead, manganese, zinc, selenium, tellurium, gallium, zirconium, indium or arsenic, one or more of an alkali metal or thallium, optionally, one or more of tin, molybdenum, boron, germanium and, optionally, one or more of calcium, strontium, magnesium, and barium, and a second composition containing bismuth, iron, molybdenum, optionally one or more of an alkali metal, samarium or silver, optionally one or more of manganese, chromium, copper, vanadium, zinc, cadmium or lanthanum, optionally one or more of phosphorus, arsenic, antimony, tellurium, tungsten, boron, tin, lead or selenium and optionally, one or more of cobalt, nickel or alkaline earth metal. In the first catalyst composition, molybdenum is present at no more than 2 atoms per atom of vanadium, bismuth is present at no more than 0.2 atoms per atom of vanadium, antimony is present at no more than 0.01 atom per atom of vanadium and the first catalyst composition is essentially uranium free.
All the above patent documents describe selective oxidation catalysts containing vanadium in combination with antimony or/and molybdenum as major constituents. Among numerous complementary elements, tungsten and bismuth are mentioned. The present invention discloses ammoxidation catalysts in which tungsten and bismuth along with vanadium are basic elements. The claimed catalysts do not contain antimony and molybdenum at all because the presence of these elements deteriorates catalyst behavior in the ammoxidation of propane to acrylonitrile.